Method of applying uniform coating by immersion



Nov. 29, 1949 R. A. LIEBEL 6 METHOD OF APPLYING UNIFORM comma BYIMMERSION Filed Aug. 21, 1945 2a 1 H 22 I i i-E-E 2-; 14

' I INVENTOR F019? 14. [iefiel HTTORNEY Patented Nov. 29, 1949 UNITEDSTATES PIA-TENT orrlcs METHOD OF APPLYING UNIFORM COATING BY IMMI'JRSIONRobert A. Liebel, Allentown, Pa.

Application August 21, 1945, Serial No. 611,885

2 Claims.

This invention relates to a method and apparatus for applying a coatingcomposition.

In a common method of coating metal sheets, plates, and wire, forinstance, the material to be coated is immersed in an enamel or otherliquid coating composition and then withdrawn through the upper exposedsurface of the composition.

The film of fiowable liquid so applied is then hardened, as byevaporation of diluting solvent initially present in the composition, bypolymerization or oxidation of the film forming material, or in somecases by cooling a composition originally softened by heat.

When the coating composition is non-metallic and consequently ofrelatively low density, as

compared to galvanizing or most metal coatings,

special problems arise. One of these is the formation of nodes or overlythick bands alternating Briefly stated, the invention comprises main-;taining the surface of the coating bath at a constantly fluctuatinglevel and constantly changing slope where the object being coatedemerges from the bath. In one embodiment, the invention com-. I prisescausing the bath of fluctuating level also 1 to be in continuous motionlaterally with respect to the surface of the object being coated. In thepreferred embodiment, the invention comprises the use of coatingcompositions of abnormally low viscosity as compared to those which havebeen with thin zones of the coating composition," ap-H pearing asstriations upon the surface of the object being coated. When, forexample, a metal sheet emerges from an enameling bath of the oleoresintype, the enamel alternately clings to and releases itself from thesheet.. The level of the liquid at the position of emergence is raisedmomentarily, then gives way, only to rise again in a moment, and thensink. This repeated alternation of lifting and sinking of the surfacegives thickened bands or nodes of the coating:

composition at levels on the object to which the surface momentarilyadhered.

This problem of the unevenness of coatingsso applied has been recognizedfor a long time. Various means have been proposed to correct the Sheetsare sometimes economical method of applying a coating that is of thedesired thickness in all areas without ob jectionable thickness in anyzone. The invention I thusavoids unevenness and unnecessary consumptionoi coating material.

used heretofore in coating objects by immersion; and withdrawal frombaths of like coating material.

Included within the invention is the herein de-- scribed apparatus forcausing the continually fluctuating level of upper surface of thecoating.

bath.

Using the method and apparatus described, I obtain a substantiallyuniform thickness of coat-I ing upon an object. When it is desired tosmooth additionally the surface of the applied fllm of coatingcomposition, heat may be applied as by i electrical strip heaters to thecoated object immediately after withdrawal from the coating bath, sothat the elevation of temperature causes reduction of viscosity of thecoating composition and promotes flow in the exposed surface of thecomposition. This feature is also included as part of the presentinvention.

The invention will be illustrated in greater detail by description inconnection with the at-' tached drawing to which reference is made.

Fig. 1 is a plan view partly in section of my improved apparatus.

' Fig. 2 is an end view in section of the apparatus, the view being online 2-2 of Fig. 1.

Fig. 3 is a sectional view on line 3-3 of Fig. 2.- The views are partlydiagrammatic. Parts not shown or described in detail are conventional 1when considered. individually.

There are shown container III for the coating bath l2. The object to becoated such as the strip of flexible steel 14 is passed over guiderollerjlli into the bath and over the second guide roller 18.-- It isthen withdrawn and passed between heaters 20.

The means for moving the strip are conventional andnot shown. Disposedon opposite sides of the sheet l4 means for setting up waves of desiredfrequency are in the surface portions of the coating composition I2. Inthe illustrative form shown, these wave-making agitators are parallelscrews 22. One of them is driven by a usual type of motor and belt driveindicated at 26 and the other through gears 24. The wavy surface of thecoating bath is indicated generally at 28 in Fig. 3.

The flexible metal sheet I4 is passed through the coating composition,and withdrawn continuously through the upper surface of the composition.When the coating composition is one which is hardenable by heat, thecoated object may be passed through a usual type of heated chamber (notshown) of proper temperature and size to cause evaporation of somevolatile solvent, polymerization, or whatever change is required tocause setting of the film of coating composition to hard, durablecondition.

As the object is withdrawn from the coating bath, the level of the bathwhich washes the surface of the object as withdrawn is caused tofluctuate constantly as by the setting up of waves or ripples at thesurface by means of the agitators 22. Care is required in the design ofthe agitators and their method of opera tion to avoid introducing'airbubbles. The agitators should be of kind to give an adequate wave effectwithout churning in air.

I have used to advantage fluctuations of the surface level of frequency20 or more a minute for speeds of movement of'the sheet it up to 300linear feet a minute. Satisfactory resultswith most objects beingcoatedare obtained when the rate of fluctuation of the surface level is madeat or more per minute and preferably at least 20 to 140 for speeds ofmovement of the sheet 14 up to 560 linear feet a minute, fluctuationsbeing figured as complete cycles from the crest of one wave to the crestof the succeeding wave.

At the frequency of 10 fluctuation-s for 500 feet of movement of thesheet, this is a ratio of l fluctuation from crest to crest of the wavefor each 50 feet of emergence'of the'sheet. When it is considered thatthe depth :of the ripples or waves 'is ordinarily about 4; inch from topto bottom of the wave or within the range of A to 1 inch, it will benoted that the recoating effect, due to the washin of the enamelcomposition up and down on the surface of the sheet being coated, is arelatively minor influence. It is considered that a major part of theeffect is due to the irregularity'of surface of the top oi. theenameling composition. There-is a constantly changing slope of surfaceof the enamel bath, that is, there is a continual change in the anglewhich-the surface of the bath at any point-- makes with the horizontal;As a result of this, there is at no time an appreciable extent of levelsurface of the composition in contact with the object being coated.

The fluctuation of the level of the coating com- I position prevents thelevel from becoming associated for any appreciable time with anyparticular spot on the article being withdrawn.

As a result there is no clinging of the surface of thecoatingcomposition-to'the object being withdrawn. There is no pullingup of thesurface followed by co1lapse.- There are no nodes of the coatingcompositionon the object coatecL- Also the lateral movement of thecoating com- The invention makes possible, in a simple and economicaloperation, the application of a thicker coating of the given compositionthan heretofore obtained by the immersion and withdrawal method.

When a thin but uniform coating is desired, then heat may be applied asby passing the coated object, at a level just above that of emergencefrom the bath, as between electrical heaters shown in the drawing at 2B.These heaters cause a rise in temperature and a decrease in viscosity ofthe adhering coating solution. They promote run back of coatingmaterial. Also the heaters exert a maximum effect upon the surface ofthe coating film and thus make this the least viscous part of the film,so as to cause smoothin of the film surface.

" may be used conventional coating compositions position of continuallyfluctuating level (:On-

the surface of the of the following types: pigmented baking enamels ofthe common oleoresin type; alkyd, phenolformaldehyde, 7 andurea-formaldehyde resin varnishes and ename1s;- and finishing composttions containing vinyl or styrene polymers of resinous consistency.These' compositions as used may be diluted, with-any conventionalsolvent or thinner for the selected composition, ,to the range ofviscosities described. Among the solvents that may be used are toluene,naphtha, The solvents seturpentine, and butyl acetate. lected should bethose that are compatible with the finishing compositions selected.

The problem of alternate pulling and release of the surface as theobject emerges is serious with the relatively light Weight coatingcompositions of the kind described. This problem is not serious withmetal coatings where the spe- The invention makes possible the use'ofcoat ing compositions of abnormally low viscosity.

Other objects that may be coated with the "method and apparatusdescribed, particularly with the light weight, non-metallic types ofcoatings, include wire, textile fabrics, woven wire gauze, paper, sheetsand webs of plastic material, and like material. In addition to suchstrip material, thereby may be dip-coated, by the present method andapparatus; rigid articles such as castings, machine or radiator covers,lamp shades, novelties and toys. The strip steel, wire, and castings areexamples of objects having anon-absorbent, smooth surface.

It will be understood from the description that has been given thatthere is relative movement between the liquid coating composition andthe object being coated, this movement being in more than one directionat the position of emergence of the object from the upper surface of thecoating composition. In the wave' motion referred to there is alwayssome lateral movement. Also there is relative movement in a verticaldirection between the object to be coated and the surface of the liquidthroughwhich the object is being withdrawn.

It will be understood also that it intended to cover all changes andmodifications of the exv5 amples of the invention herein chosen for theHowever,

purpose of illustration which do not constitute departures from thespirit and scope of the invention.

What I claim is:

1. The method of applying a coating of a liquid heat hardenable organicfinishing composition of low viscosity to an object having anon-absorbent smooth surface which comprises immersing -the object inthe said composition, withdrawing the object at a steady rate throughthe upper surface of the composition, and subjecting said composition tomechanical agitation applied at and immediately below the surfacethereof, to cause alternations of rise and fall of the level of the saidsurface at predetermined regular frequency of at least complete cycles aminute and to cause also movement of the coating liquid at the saidsurface in direction generally lateral to the surface of the objectbeing withdrawn through the surface.

6 2. The method described in claim 1, the frequency of said fluctuationbeing at least one for each feet of linear movement of the object beingcoated.

ROBERT A. LIEBEL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

